The present invention relates to a mass spectrometer and a method of mass spectrometry.
A tandem mass spectrometer is known which comprises an ion source, a mass filter which is arranged to transmit parent ions having a particular mass to charge ratio, a fragmentation cell arranged downstream of the mass filter which is arranged to fragment the parent ions transmitted by the mass filter, and a mass analyser which is arranged to mass analyse the fragment ions produced in the fragmentation cell. The fragmentation cell comprises a chamber wherein parent ions are arranged to undergo energetic collisions with gas molecules. However, the energetic collision of parent ions with gas molecules can cause parent ions to become scattered and this can cause parent ions to become lost prior to fragmentation. Fragment or product ions produced within the fragmentation cell may also become lost due to scattering effects. This can have the effect of lowering sensitivity.
It is known that an inhomogeneous RF electric field will direct ions to regions where the RF electric field is weakest. This characteristic is exploited in RF ion guides where the background gas pressure is sufficient to cause a significant number of ion-molecule collisions. A known RF ion guide comprises a plurality of rod electrodes arranged parallel to a central axis. An RF voltage is applied between neighbouring electrodes. The resulting radial RF electric field is weakest along the central axis and hence ions which are scattered as a result of ion-molecule collisions will tend to be re-directed back to the central axis of the RF ion guide. As a result ions are confined within the RF ion guide.
The known RF ion guide is commonly provided in the collision cell of a tandem mass spectrometer and selected parent or precursor ions are arranged to undergo collisions with gas molecules within the collision cell. The known RF ion guides have been shown to transmit ions with high efficiency in spite of ions undergoing a large number of collisions with background gas molecules.
The most common form of tandem mass spectrometer is known as a triple quadrupole mass spectrometer. A triple quadrupole mass spectrometer comprises an ion source, a first quadrupole mass filter, a gas collision cell comprising an RF quadrupole rod set ion guide, and a second quadrupole mass filter. Other arrangements are known wherein the collision cell may comprise a hexapole or octopole rod set ion guide or an ion tunnel ring stack ion guide.
The transmission characteristics of a RF ion guide is known to vary with the mass to charge ratio of the ions. For a given geometrical configuration and a given RF voltage and frequency there will be a range of mass to charge ratio values for which the radial confinement of the ions is relatively high and consequently the ion transmission efficiency is also relatively high. However, outside of this range the overall transmission efficiency of ions will be reduced.
The maximum instantaneous velocity of ions having relatively low mass to charge ratios is higher than that of ions having relatively high mass to charge ratios. As a consequence, ions having relatively low mass to charge ratios will follow trajectories with relatively large radial excursions and ions having mass to charge ratios below a certain critical value may strike the electrodes of the RF ion guide and hence become lost to the system. The critical mass to charge ratio below which ions may be lost in this way is generally known as the low mass to charge ratio cut off value. The ion transmission efficiency drops off rapidly for ions having mass to charge ratios below the low mass to charge ratio cut off value.
In a conventional gas collision cell ions undergo multiple energetic collisions with background gas molecules in order to induce fragmentation. Ions which are scattered due to these energetic collisions are confined about the central axis of the RF ion guide in spite of this scattering process. However, for a given RF voltage and frequency the time averaged or effective radial confining force due to the inhomogeneous RF field decreases with mass to charge ratio. As a consequence, ions having relatively high mass to charge ratios and which are scattered are less effectively confined by the RF ion guide and the ion transmission efficiency starts to decrease with increasing mass to charge ratio. In this case the ion transmission efficiency drops off only gradually with increasing mass to charge ratio value.
As a consequence of these two considerations there is an optimum range of RF voltages for a given RF frequency and geometrical configuration of the RF ion guide for which energetic ions are efficiently transmitted through and radially confined within the gas collision cell. Alternatively, for a given RF voltage and frequency and a given geometrical configuration of the RF ion guide, there is a limited range of mass to charge ratios for which energetic ions are efficiently transmitted through the gas collision cell.
A problem with a conventional gas collision cell is that parent or precursor ions which initially enter the collision cell will have a first relatively high mass to charge ratio whereas the resulting product or fragment ions formed in the gas cell (and which subsequently exit the gas collision cell) will have a second relatively low mass to charge ratio. If the mass to charge ratios of the parent or precursor ions and the product or fragment ions are substantially different, then the optimum range of RF voltages required for efficient transmission of the two different groups of ions will be substantially different and the two ranges may not overlap. As a result, neither the parent or precursor ions nor the product or fragment ions will be transmitted with high efficiency.
It is desired to provide an improved mass spectrometer.